Tetrafluorammonium tetrafluoroborate (NF.sub.4 BF.sub.4) is used as a common ingredient in formulations for oxidizer source solid gas generators which find particular utility in combustion driven chemical lasers and other oxidation applications.
It has been demonstrated that tetrafluorammonium tetrafluoroborate may be prepared by metathesis of NF.sub.4 SbF.sub.6 with CsF or AgF in anhydrous hydrogen fluoride to yield CsSbF.sub.6 or AgSbF.sub.6 as a precipitate and tetrafluorammonium bifluoride which is in turn reacted with boron fluoride to yield tetrafluorammonium tetrafluoroborate. The net reactions for cesium fluoride, for instance, may be written as ##STR1## Removal of the solid precipitate and treatment of the liquid phase which BF.sub.3 forms the BF.sub.4 salt by the reaction ##STR2## Excess hydrogen fluoride is removed by evaporation.
Subject to strict safety precautions, NF.sub.4 AsF.sub.6 may also be reacted with RbF to yield RbAsF.sub.6 as the precipitate as a predicate to forming NF.sub.4 BF.sub.4 from NF.sub.4 HF.sub.2.
The main deficiency in the procedure is the cost of the added metal fluoride. The present cost of cesium fluoride is about $15,000 per pound mole and the cost of silver fluoride $15,500 per pound mole.
Moreover, there is always a solid residue of the reactions which occur in the use of a heavy metal fluoride compound as an oxider. It would be desirable to provide an oxider source which does not leave such a residue.